Scrubbers are extensively employed in chemical operations to scrub gas streams of undesirable components before releasing into the atmosphere. Some of the scrubbers employ a solution containing a compound that can react with the target component in the gaseous stream, thereby enhancing the efficiency of absorption multifold. In the polycarbonate industry, a solution of caustic is often used to scrub phosgene off the gas stream. Caustic solution, as described herein, describes an aqueous solution comprising sodium hydroxide (NaOH) that may or may not contain sodium salts such as NaCl and Na2CO3 as well as ionic impurities typically found in hard water such as salts of Ca2+ and Mg2+, and trace amounts of organic or inorganic contaminants. Caustic solution shows a very high reactivity towards phosgene that enhances the efficiency of these scrubbers. For instance, caustic scrubbers are used to neutralize residual phosgene in enclosure vent gas in a phosgene production process. The spent caustic stream out of the caustic scrubbers is either fed to another scrubber which is daisy chained to it or wasted to the wastewater treatment plant. The effluent out of the scrubbers contains not only unused caustic but also a significant concentration of sodium carbonate (Na2CO3), which makes it hard to recover caustic for recycle back into the process. Carbonates have a limited solubility in water or in aqueous caustic solutions and may precipitate out of the solution. Hence, any method to recover the useful caustic from the waste stream by recycling it back to the scrubber must remove carbonates from the stream to prevent them from building up in the recycle stream and adversely affecting the unit operation by precipitating out of solution.
Chemical methods to precipitate carbonates out of aqueous caustic solution by employing calcium oxide are known. These methods are recommended only for moderately concentrated caustic solutions (less than 10 molar (<10 M)) since the solids produced in case of highly concentrated caustic solutions (greater than 10 molar (>10 M)) are very fine and filtration of the slurry is problematic. One of the main drawbacks of a chemical precipitation method is that it ends up recycling soluble CaCO3 back to the scrubber, which in due time will precipitate out as the concentration builds up in the recycle stream and plug the scrubbers. Additionally, since CaCO3 is not highly soluble in water, the scrubbers will need to be acidized frequently for efficient operation. These factors make this method unattractive in industrial practice as the savings in caustic usage are neutralized by the cost of calcium oxide and increased cost of maintenance.
Thus there is a need for a method to remove carbonates from a caustic scrubber waste stream to allow the treated caustic waste stream to be recycled.